First identified in the 1980s, chronic fatigue syndrome (CFS) is an illness marked by severe fatigue that lasts more than six months. Between one and four million people in the United States — about 2.5 percent of the population — have chronic fatigue symptoms, which can cause them to struggle through the day with only about half of their previous energy levels. Also called chronic fatigue and immune dysfunction syndrome, CFS is sometimes the result of other health conditions.

To get a chronic fatigue syndrome diagnosis, you must first have severe fatigue that has lasted six months or more, and your doctor must have ruled out other possible medical causes, such as a sleep disorder, anemia, or depression. (While someone can have depression and chronic fatigue syndrome at the same time, the doctor must rule out fatigue solely associated with depression.)

A feeling of being more unwell than usual — called malaise — for about 24 hours after exercise

With chronic fatigue syndrome, severe tiredness occurs before the other symptoms, although many people report a flu-like illness just before the long-term fatigue sets in. Chronic fatigue syndrome is also sometimes associated with anxiety, irritable bowel syndrome, and fibromyalgia.

What Causes Chronic Fatigue Syndrome?

Although many researchers are studying chronic fatigue syndrome, no research has been able to identify a single cause of CFS. Rather, scientists' theories that chronic fatigue syndrome could be caused by one or more of the following:

Infections. A variety of viruses, including Epstein Barr virus, some types of herpes viruses, and most recently the retrovirus XMRV, been linked to chronic fatigue syndrome.

Genetics. Some people may be more susceptible because of an inherited risk.

Neuroendocrinology. A complex interaction between neurotransmitters and hormones could be at the root of chronic fatigue syndrome.

Trauma. The physical stress related to surviving traumatic events has also been identified as a possible contributing factor.

What Chronic Fatigue Syndrome Feels Like

Chronic fatigue symptoms differ between patients, but severe fatigue or chronic tiredness that interferes with work or daily activities is common. Patients may also complain of pain and achiness and a kind of brain fog that makes it hard to concentrate or remember recent events or details.

If you've been exhausted for months, can't carry out your daily activities, and nothing you have tried makes you feel better, it's time to see a doctor. Studies suggest that close to 80 percent of people who are living with chronic fatigue syndrome have never been formally diagnosed and are not getting the treatment they need, but data also implies that getting a diagnosis early in the course of the disease — before you've had it for two years — gives you the best chance of improving your symptoms through treatment.